Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.     

Quantum interference in radial heterostructure nanowires

Core/shell heterostructure nanowires are one of the most interesting mesoscopic systems potentially suitable for the study of quantum interference phenomena. Here, we report on experimental observations of both the Aharonov-Bohm (h/e) and the Altshuler-Aronov-Spivak (h/2e) oscillations in radial core/shell (In2O3/InOx) heterostructure nanowires. For a long channel device with a length-to-width ratio of about 33, the magnetoresistance curves at low temperatures exhibited a crossover from low-field h/2e oscillation to high-field h/ e oscillation. The relationship between the oscillation period and the core width was investigated for freestanding or substrate-supported devices and indicated that the current flows dominantly through the core/shell interface.     

Gold-nanoparticle-functionalized In₂O₃ nanowires as CO gas sensors with a significant enhancement in response

We present the room-temperature sensing of gold nanoparticle (AuNP)-functionalized In(2)O(3) nanowire field-effect transistor (NW-FET) for low-concentration CO gas. AuNPs were functionalized onto In(2)O(3) nanowires via a self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS-SAM). The nanowires were mounted onto the Au electrodes with both ends in Schottky contacts. High sensor response toward low concentration of CO gas (200 ppb-5 ppm) at room temperature is achieved. The presence of AuNPs on the surface of In(2)O(3) nanowire serves to enhance the CO oxidation due to a higher oxygen ion-chemisorption on the conductive AuNP surfaces. Detailed studies showed that the sensing capabilities were greatly enhanced in comparison to those of bare nanowires or low coverage of Au NP-decorated nanowires. When the sensor is exposed to CO, the CO molecules interact with the preadsorbed oxygen ions on the AuNP surface. The CO oxidation on the AuNPs leads to the transfer of electrons into the semiconducting In(2)O(3) nanowires and this is reflected as the change in conductance of the NW-FET sensor. This work provides a promising approach for fabricating nanowire devices with excellent sensing capabilities at room temperature.     

Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.     

Conductometric chemical sensor based on individual CuO nanowires

CuO nanowires with high crystalline quality are synthesized via a simple thermal oxidation method. Charge conduction on individual nanowires under a transverse electric field exhibits an intrinsic p-type semiconducting behavior. Variations in signal transducer in different chemical gas environments are measured on individual CuO nanowire field effect transistors. They demonstrate good performance to both NO(2) and ethanol gasses. In particular, the nanowire chemical sensor reveals a reverse response to ethanol vapor under temperature variation. Experimental results and first-principles calculations indicate that ethanol is oxidized in air at high temperature, resulting in the production of CO(2) and H(2)O. The strong H(2)O adsorption leads to the reversal behavior, due to the electron transfer from H(2)O molecules to the CuO surface.     

Nanowire lithography: fabricating controllable electrode gaps using Au-Ag-Au nanowires

A method to fabricate nanowire electrodes possessing controllable gaps is described. The method relies on electrochemical deposition and selective chemical etching or heating to selectively remove the Ag segment of Au-Ag-Au nanowires. Because the thickness of the Ag segment directly dictates the size of the nanogap, the gap width can be easily controlled during the nanowire fabrication process. Herein, we demonstrate gaps with 2 microm, 100 nm and 20 nm widths via the above-mentioned approaches. In addition, we observed that small gaps (approximately 20 nm) can be formed through annealing Au-Ag-Au nanowires at 200 degrees C in air. Electrical contact between nanowire electrodes and contact pads is studied. Using nanowire electrodes with a 100 nm gap, we subsequently fabricate organic field effect transistors (FETs) with regioregular poly(3-hexylthiophene).     

A radio frequency single-electron transistor based on an InAs/InP heterostructure nanowire

We demonstrate radio frequency single-electron transistors fabricated from epitaxially grown InAs/InP heterostructure nanowires. Two sets of double-barrier wires with different barrier thicknesses were grown. The wires were suspended 15 nm above a metal gate electrode. Electrical measurements on a high-resistance nanowire showed regularly spaced Coulomb oscillations at a gate voltage from -0.5 to at least 1.8 V. The charge sensitivity was measured to 32 microe rms Hz(-1/2) at 1.5 K. A low-resistance single-electron transistor showed regularly spaced oscillations only in a small gate-voltage region just before carrier depletion. This device had a charge sensitivity of 2.5 microe rms Hz(-1/2). At low frequencies this device showed a typical 1/f noise behavior, with a level extrapolated to 300 microe rms Hz(-1/2) at 10 Hz.     

Controlled synthesis of NiS nanoparticle/CdS nanowire heterostructures via solution route and their optical properties

In the present study, we have successfully synthesized the novel heterostructure of NiS nanoparticle (NP)/CdS nanowire (NW) through solution approach. The first step, CdS nanowires were synthesized by a convenient solvothermal route. Then, NiS nanoparticles were grown on the surface of CdS nanowires in a chemical solution of NiCl₂·6H₂O and anhydrous ethanol at 200 °C. The new catalyst-assisted growth mechanism of the NiS NP/CdS NW heterostructure has been tentatively discussed on the basis of experimental results. A detailed study of the effect of experimental parameters, such as reaction time, reaction temperature, and reaction solvent are also studied. The as-prepared products are characterized by field-emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and their optical properties are measured by Raman spectra and PL spectra. Furthermore, using CdS nanowires and NiS NP/CdS NW heterostructure as examples, our study suggests that this general method can be employed for construction of other semiconductor heterostructures with novel properties.     

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.     

Growth of GaInAs/AlInAs heterostructure nanowires for long-wavelength photon emission

We investigated the growth of GaInAs/AlInAs heterostructure nanowires on InP(111)B and Si(111) substrates in a metalorganic vapor phase epitaxy reactor. Au colloids were used to deposit Au catalysts 20 and 40 nm in diameter on the substrate surfaces. We obtained vertical GaInAs and AlInAs nanowires on InP(111)B surfaces. The GaInAs nanowires capped with GaAs/AlInAs layers show room-temperature photoluminescence. The peak exhibits a blue-shift when the Ga content in the core GaInAs nanowire is increased. For the GaInAs/AlInAs heterostructure growth, it is possible to change the Ga content sharply but Al also exists in the GaInAs layer regions. We also found that the ratios of Ga and Al contents to In content tend to increase and the axial growth rate to decrease along the nanowire toward the top. We were also able to make vertical GaInAs nanowires on Si(111) surfaces after a short growth of GaP and InP.     

Rayleigh-instability-induced metal nanoparticle chains encapsulated in nanotubes produced by atomic layer deposition

Cu nanoparticle chains encapsulated in Al2O3 nanotubes were successfully generated in a controlled manner by reduction of CuO nanowires embedded in Al2O3 at a sufficiently high temperature. The Al2O3 coating was deposited by atomic layer deposition (ALD). The particles mainly show a rodlike shape and are regularly distributed. The particle diameters and chain lengths corresponding to the inner diameters and lengths of the tubes, respectively, are controlled by the size of the CuO nanowire templates. Rayleigh instability, assisted by the uniform volume shrinkage created by the reduction of oxide to metal, is proposed to induce the formation of the nanochains. This method may potentially be extended to the synthesis of nanochains of other metals by reducing corresponding oxide nanowires embedded in ALD shells.     

Thermal stability and strength of polycrystalline nanowires

We considered a polycrystalline cylindrical nanowire with initial radius R₀ composed of identical cylindrical grains of the length L₀, strained uniaxially by an external stress P. At the temperatures at which some surface and grain boundary diffusion are allowed the thinning of the nanowire in the vicinity of grain boundaries occurs due to the phenomenon of grain boundary grooving. We calculated the equilibrium shapes of the nanowire achieved after long annealing times. Our calculations demonstrated that for any given L₀/R₀ ratio some critical value of the applied stress exists above which the nanowire is unstable and breaks down into the string of isolated spherical particles, in full analogy with the Rayleigh instability of long cylinders. The kinetics of the shape change was calculated numerically. It was shown that the rate of thinning of unstable nanowires diverges as the moment of breakdown is approached. We also demonstrated that the breakdown may occur even for nominally stable nanowires “on the way” to achieving their equilibrium shape. Therefore, the stability of nanowire is determined by a combination of geometric (L₀/R₀), thermodynamic (grain boundary energy), and kinetic (ratio of grain boundary and surface diffusivities) parameters. An application of external tensile stress accelerates the breakdown of nanowires.     

Long, highly-ordered high-temperature superconductor nanowire arrays

The preparation and electrical properties of high-temperature superconductor nanowire arrays are reported for the first time. YBa2Cu3O(7-delta) nanowires with widths as small as 10 nm (much smaller than the magnetic penetration depth) and lengths up to 200 microm are studied by four-point electrical measurements. All nanowires exhibit a superconducting transition above liquid nitrogen temperature and a transition temperature width that depends strongly upon the nanowire dimensions. Nanowire size effects are systematically studied, and the results are modeled satisfactorily using phase-slip theories that generate reasonable parameters. These nanowires can function as superconducting nanoelectronic components over much wider temperature ranges as compared to conventional superconductor nanowires.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

High-performance transparent conducting oxide nanowires

We report the growth and characterization of single-crystalline Sn-doped In2O3 (ITO) and Mo-doped In2O3 (IMO) nanowires. Epitaxial growth of vertically aligned ITO nanowire arrays was achieved on ITO/yttria-stabilized zirconia (YSZ) substrates. Optical transmittance and electrical transport measurements show that these nanowires are high-performance transparent metallic conductors with transmittance of approximately 85% in the visible range, resistivities as low as 6.29 x 10(-5) Omega x cm and failure-current densities as high as 3.1 x 10(7) A/cm2. Such nanowires will be suitable in a wide range of applications including organic light-emitting devices, solar cells, and field emitters. In addition, we demonstrate the growth of branched nanowire structures in which semiconducting In2O3 nanowire arrays with variable densities were grown epitaxially on metallic ITO nanowire backbones.     

Bipolar Resistive Switching of Single Gold-in-Ga(2)O(3) Nanowire

We have fabricated single nanowire chips on gold-in-Ga(2)O(3) core-shell nanowires using the electron-beam lithography techniques and realized bipolar resistive switching characteristics having invariable set and reset voltages. We attribute the unique property of invariance to the built-in conduction path of gold core. This invariance allows us to fabricate many resistive switching cells with the same operating voltage by simple depositing repetitive metal electrodes along a single nanowire. Other characteristics of these core-shell resistive switching nanowires include comparable driving electric field with other thin film and nanowire devices and a remarkable on/off ratio more than 3 orders of magnitude at a low driving voltage of 2 V. A smaller but still impressive on/off ratio of 10 can be obtained at an even lower bias of 0.2 V. These characteristics of gold-in-Ga(2)O(3) core-shell nanowires make fabrication of future high-density resistive memory devices possible.     

Effect of Oxide Assisted Metal Nanoparticles on Microstructure and Morphology of Gallium oxide Nanowires

Several researches have been reported about the characteristic of β-Ga2O3 nanowires which was synthesized on nickel oxide particle. But indeed, recent researches about synthesis of β-Ga2O3 nanowires on oxide-assisted transition metal are limited to nickel or cobalt oxide catalyst. In this work, Gallium oxide (β-Ga2O3 ) nanowires were synthesized by a simple thermal evaporation method from gallium powder in the range of 700 - 1000℃ using the iron, nickel, copper, cobalt and zinc oxide as a catalyst, respectively. The β-Ga2O3 nanowires with single crystalline without defects were successfully synthesized at the reaction temperature of 850, 900 and 950℃ in all the catalysts. But optimum experimental condition in synthesis of nanowires varied with the kind of catalyst. As increasing synthesis temperature,the morphology of gallium oxide nanowires changed from nanowires to nanorods, and its diameter increased. From these results, we could be proposed that the growth mechanism of β-Ga2O3 nanowires was changed with synthesis temperature of nanowires. Microstructure and morphology of Synthesized nanowire was characterized by HR-TEM, FE-SEM, EDX and XRD.     

Size dependent magnetization and high-vacuum annealing enhanced ferromagnetism in Zn(1-x)Co(x)O nanowires

Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn(1-x)Co(x)O nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn(1-x)Co(x)O nanowires displayed weak ferromagnetism while the high-vacuum annealed nanowires exhibited strong ferromagnetic ordering at room temperature. Size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. Field cooled and zero-field cooled magnetization and coercivity measurements were performed between 2 and 300 K to study the evolution of magnetism from the weak to the strong ferromagnetic states. In particular, superparamagnetic features were observed and shown to be intrinsic characteristics of the DMS Zn(1-x)Co(x)O nanowires. The room-temperature spontaneous magnetization of individual Zn(1-x)Co(x)O nanowires was also established by using magnetic force microscope measurements.     

Growth mechanism and elemental distribution of beta-Ga2O3 crystalline nanowires synthesized by cobalt-assisted chemical vapor deposition

Long beta-Ga2O3 crystalline nanowires are synthesized on patterned silicon substrates using chemical vapor deposition technique. Advanced electron microscopy indicates that the as-grown beta-Ga2O3 nanowires are consisted of poly-crystalline (Co, Ga)O tips and straight crystalline beta-Ga2O3 stems. The catalytic cobalt not only locates at the nanowire tips but diffuses into beta-Ga2O3 nanowire stems several ten nanometers. A solid diffusion growth mechanism is proposed based on the spatial elemental distribution along the beta-Ga2O3 nanowires at nanoscale.     

Fabrication of suspended silicon nanowire arrays

A method to fabricate suspended silicon nanowires that are applicable to electronic and electromechanical nanowire devices is reported. The method allows for the wafer-level production of suspended silicon nanowires using anisotropic etching and thermal oxidation of single-crystal silicon. The deviation in width of the silicon nanowire bridges produced using the proposed method is evaluated. The NW field-effect transistor (FET) properties of the device obtained by transferring suspended nanowires are shown to be practical for useful functions.